Alkyl esters of fatty acids, produced between a fat or oil, which is an esterified product of glycerol and a fatty acid, and an alkyl alcohol, not only have been used as industrial raw materials in the fields of cosmetic and medicaments, but also have remarked increasing interests in the recent years as substitute fuels for fuel oils so-called bio-diesel fuels (hereinafter referred to as “BDF”), from the viewpoint of the prevention of global warming and the reduction in environmental loads.
Regarding methods for producing alkyl esters of fatty acids from a triglyceride, a main component of a fat or oil, and an alkyl alcohol by a transesterification reaction, a plural methods have been conventionally known. An industrially most well used method includes the steps of using an alkali metal hydroxide such as potassium hydroxide as a homogeneous catalyst, and carrying out a transesterification reaction near a boiling point of the alcohol in the presence of the catalyst. However, in this this method, there are some disadvantages that the solubility of the catalyst in an organic solvent is high, so that the catalyst is dissolved in an alkyl ester of a fatty acid in a high concentration, whereby a complicated procedure such as rinsing with water is necessitated for the production of a high-quality BDF having a high purity, and a disadvantage in the post-treatment of an alkali-containing wastewater, or the like.
As a method for avoiding these disadvantages, for example, Patent Publication 1 (Japanese Patent Laid-Open No. 2000-109883) discloses a method including the step of reacting an alcohol in a supercritical state and a fat or oil. In this method, a purification procedure, such as removal of a catalyst is not necessitated because the reaction is carried out in the absence of a catalyst. However, since the reaction is carried out under high-temperature and high-pressure conditions, this reaction is not suitable for practical use energetically or process cost-wise.
In addition, there is a method of simplifying the purification procedures by using a fixed catalyst, not a homogeneous catalyst. For example, Patent Publication 2 (Japanese Patent Laid-Open No. Hei 6-313188) discloses a method in which a cationic exchange resin, a composite metal oxide, a solid heteropoly acid or the like is used as a solid acid catalyst. Generally, however, an acid catalyst has a lower activity in the transesterification reaction than a base catalyst; therefore, there are some disadvantages that the reaction must be carried out at a high temperature or for a long period of time, whereby an ether or a degraded product or the like is formed as a by-product.
As a method of using a solid base catalyst as a fixed catalyst, for example, Patent Publication 3 (Japanese Patent Laid-Open No. 2001-271090) discloses a method for producing an ester under conditions within a temperature range of from 90° to 240° C. in the presence of a solid catalyst including calcium hydroxide or calcium oxide; and Patent Publication 4 (Japanese Patent Laid-Open No. 2004-35873) discloses a method for reacting a fat or oil (including a waste oil) and an alcohol with quick lime or dolomitic lime. However, calcium hydroxide or calcium oxide generally has a low activity in the transesterification reaction; therefore, there is a disadvantage that the reaction must be carried out at a high reaction temperature, or for a long reaction time, that a catalyst is needed in a large amount, or the like.
Patent Publication 5 (Japanese Patent Laid-Open No. Hei 5-286904) discloses a method for producing an aromatic ester of an aromatic carboxylic acid including the steps of pre-treating calcium oxide with an alcohol for the reaction to give a catalyst an activated structure of the formula Ca(OH)(OR) . . . , and carrying out a transesterification reaction with an aromatic alcohol using the activated catalyst as a solid base catalyst. However, this Patent Publication 5 is completely silent on the production of an alkyl ester of a fatty acid that can be used as BDF.    Patent Publication 1: Japanese Patent Laid-Open No. 2000-109883    Patent Publication 2: Japanese Patent Laid-Open No. Hei 6-313188    Patent Publication 3: Japanese Patent Laid-Open No. 2001-271090    Patent Publication 4: Japanese Patent Laid-Open No. 2004-35873    Patent Publication 5: Japanese Patent Laid-Open No. Hei 5-286904